In the working environment of a modern vehicle an ever-increasing amount of information can be made accessible to the driver. At the same time, almost all the available space of the driver's cab of a typical commercial vehicle is taken up by various devices and equipment, which makes it difficult to find a place to display the information. Providing a large, continuous display surface in front of the driver is impossible, since it would be partly concealed by the steering wheel, it would obstruct the view and distract the driver.
In the case of passenger cars, attempts have previously been made to reflect an image in the plane of the windscreen. Devices of this type are usually referred to as “head-up displays”, because they allow the driver to read the information without looking down on the instrument panel. In a conventional head-up display, the unit creating the image is located in or adjacent to the instrument panel, i.e. in front of and obliquely below the driver, and the image is projected upwards onto the windscreen by way of mirrors and other optical elements. On the windscreen, which may have been treated with a reflection-enhancing material, a virtual image is then formed which provides the driver with the information displayed. One example of such a display is disclosed in WO 89/03059.
When the real image, as shown in WO 89/03059, is created obliquely below and in front of the driver, it is difficult, impossible even, to avoid the virtual image being created somewhere in the active field of vision used by the driver to survey the traffic situation. For this reason, the reflecting surface, which is thus located in the windscreen, is transparent so as not to obstruct the driver's view too much. Nevertheless, the virtual image may still interfere with the driver's view. Moreover, the transparent reflecting surface is such that the quality of the information displayed will be low, and only simple visual indications can be displayed.
A better solution is disclosed in WO 91/00674, in which an image source is arranged in the ceiling of a passenger car, and a mirror is arranged slightly above the driver's field of vision for the purpose of reflecting information generated by the image source. A similar solution is disclosed in WO 94/08264. To reduce the need for the driver to significantly adapt the eye focus when alternately looking at the area in front of the vehicle and the virtual image, it is advantageous for the reflected image to be located as far away from the driver as possible. In WO 91/00674, the mirror consisting of a holographic optical element achieves this.
Alternatively, the virtual image may be moved further away from the driver by extending the optical path between the real image and the mirror located adjacent the windscreen. In U.S. Pat. No. 5,731,903, this has been achieved by way of a complex system of mirrors. The device shown is not a head-up display in the strict sense of the word, since the virtual image is not projected onto the windscreen but displayed on a mirror recessed in the instrument panel. However, it is apparent from the specification that it is both time-consuming and expensive to provide a device featuring a long optical path, which can also be contained in an instrument panel.